The present invention relates to a fault detection apparatus for a purge system supplying (purging) fuel vapor from a fuel tank to an intake system of an engine.
Vehicles typically carry a purge system for purging fuel vapor from a fuel tank to an engine intake passage. The purge system includes a canister for collecting fuel vapor from the fuel tank, a vapor passage connecting the fuel tank with the canister, and a purge passage connecting the canister with an intake passage. Fuel vapor in the canister is supplied-to the intake passage through the purge passage.
If there is a hole or a crack in the purge passage, fuel vapor leaks from the purge system. A test apparatus exists for detecting fuel vapor leakage from the purge system. For example, Japanese Unexamined Patent Publication No. 8-240161 describes an apparatus that lowers the pressure in the purge system and then monitors pressure changes in the purge system to detect fuel vapor leakage.
Such an apparatus performs testing only when certain preconditions are fulfilled to improve the accuracy of fault detection. The preconditions include that the velocity of a vehicle is stable, the cumulative value of the pressure change in the fuel tank is within a certain values and the amount of fuel vapor generated in the fuel tank within a certain time is small (that is, the pressure change in the fuel tank within a certain time is small). However, when a vehicle is actually running, the velocity of the vehicle frequently changes. Also, fuel in the tank is shaken when the vehicle runs on a rough road, which increases the amount of fuel vapor generated in the tank. Accordingly, the preconditions are seldom fulfilled when the vehicle operates. As a result, the frequency of performing the testing is relatively low.
In recent years, there has been a need to improve the testing performance. For example, it is necessary to quickly and accurately detect holes and cracks having a maximum dimension of at least about 0.5 mm (hereinafter called 0.5 mm holes). The necessary preconditions for detecting 0.5 mm holes are more strict than those for detecting holes having a maximum dimension of at least about 1.0 mm (hereinafter called 1.0 mm holes). Therefore, the preconditions for detecting 0.5 mm holes are less frequently fulfilled, which reduces the frequency of performing the detection.
The prior art apparatus lowers the pressure in the purge system before detecting 1.0 mm holes. Then, when no 1.0 mm hole is found, detection of 0.5 mm holes is performed. Accordingly, when there is a 0.5 mm hole in the purge system, the pressure in the purge system gradually changes because of fuel vapor leakage through the hole. Therefore, it takes a relatively long time before the pressure in the purge system is stable and the preconditions for detecting 0.5 mm holes are fulfilled. If the pressure in the purge system changes during the 0.5 mm holes detection, the detection is cancelled. As a result, the detection of 0.5 mm holes is less frequently performed.
An objective of the present invention is to provide a testing apparatus for accurately and quickly detecting a small leakage in a purge system.
To achieve the above objective, the present invention provides a method for testing a purge system that purges fuel vapor in a fuel tank to an intake passage of an engine through a system passage connected with the fuel tank. The method includes changing the pressure in the system passage to a first predetermined pressure; sealing the system passage; monitoring the change of the pressure in the sealed system passage and detecting leakage of fuel vapor from the system passage; and restoring the pressure of the system passage when a testing is demanded again after the sealing step.
The present invention further provides a testing apparatus for a purge system that purges fuel vapor in a fuel tank to an intake passage of an engine through a system passage connected with the fuel tank. The apparatus is structured as follows. A pressure sensor detects the pressure in the system passage. A first valve sets the pressure in the system passage to a predetermined value. A second valve for seals the system passage. A controller monitors a signal from the pressure sensor and controls the first and second valves. The controller sets the pressure in the system passage by controlling the first and second valves when the pressure in the system passage is stable for a predetermined period. Then the controller seals the system passage and detects leakage of fuel vapor from the system passage by monitoring the change of the pressure in the sealed system passage. The controller restores the pressure in the system passage when a testing is demanded again after the sealing step.
Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.